A charged particle beam apparatus typically employs a scanning electron microscopy (SEM), which is a known technique used in semiconductor manufacturing. Defects can occur on the mask or wafer during the semiconductor fabrication process. These defects impact yield to a great degree. Defect inspection systems and defect review systems are two significant ways to monitor semiconductor yield management.
A defect inspection system detects particles, pattern defects, and process-induced defects, and typically detection results from the inspection systems are fed to defect review systems. A defect review system further analyzes defect root causes. For most of the charged particle systems, imaging resolution (which determines the detectable defect size) and throughput (which determine how many defects can be detected out in unit time) are always major concerns during sample inspection and defects review. In a defect review system, stereo imaging detection for topography analysis (which tells whether defects are protrusion or depression) is also a required function. FIG. 1 is an inspection column disclosed by Petrov et al. in U.S. Pat. No. 7,067,807 which is incorporated by reference as if fully set forth herein, and FIG. 2 is a high resolution scanning electron microscope (SEM) disclosed by Todokoro et al. in U.S. Pat. No. 5,872,358 which is incorporated by reference as fully set forth herein.
One of the common goals of all imaging systems consists of increasing the image resolution. Image resolution depends principally on the spot size of the electron beam impinging onto the sample. In order to reduce the spot size of the electron beam up to nanometers, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Electron optics produces smaller aberrations when the electrons move with higher kinetic energy. However, radiation damage to the sample is another subject to consider while utilizing high kinetic energy electron beam. Therefore the electron beam is decelerated just prior to impinging onto the sample surface.
Throughput depends principally on how fast the imaging spot can be scanning and how large the field of view is to be scanning through. Electron optics produces large off-axis aberrations when the field of view becomes large. Comparatively, magnetic scanning deflectors produce smaller deflection off-axis aberrations but can not scan with higher speed; electrostatic scanning deflectors produce larger deflection off-axis aberrations but can scan with higher speed.
A stereo imaging detection usually is realized by using side detectors as well as an in-lens detector. FIGS. 3a and 3b illustrate two typical side detector arrangements. FIG. 3a shows an arrangement of a side detector which needs a larger working distance and therefore limits the magnetic objective structure which can not produce a strong magnetic field immersion on sample surface, the imaging aberrations are increased due to increasing working distance. FIG. 3b is an arrangement which needs to use pre-lens scanning deflection; however, this arrangement has a large off-axis imaging aberrations.